Update sbt-mdoc to 2.6.4 (#1817) #2427

Job	Run time
conformance	4m 32s
build (2_12, e2e)	3m 37s
build (2_12, test)	11m 50s
build (2_13, e2e)	4m 22s
build (2_13, test)	10m 54s
build (3, e2e)	5m 13s
build (3, test)	8m 54s
build (2_12, examples_and_formatting)	3m 26s
build (2_13, scala3_compat_test)	9m 14s
build (3, scala3_compat_test)	8m 34s
ci-passed	1s
	1h 10m 37s

Provide feedback